1. Field of the Invention
The present invention is directed to a method and apparatus for selectively reinforcing detected seismic waves traveling in a first direction and attenuating detected seismic waves traveling in a second opposite undesired direction. The invention is particularly directed to reinforcing seismic waves traveling in a body of water in an ascending direction.
2. Discussion of the Prior Art
Ocean bottom seismometry is a technique for recording seismic waves by means of receivers (hydrophones) which are located at or near an ocean bottom. Typically, a single receiver, is placed at or near the ocean bottom where it detects seismic reflection and/or refraction waves passing in an ascending direction from the ocean bottom towards the surface of the water. The reflection and/or refraction waves are created when an acoustic source, disposed at or below the water surface, is excited to generate an acoustic wave which travels downward through the water and ocean floor until it is reflected and/or refracted by subsurface formation boundaries, causing reflection and/or refraction of at least a portion of the wave back toward the water surface. When the ascending waves reach the water surface, they are again partially reflected, with a 180.degree. phase change, and begin a downward movement through the water.
Because of the surface reflection, a seismic wave receiver positioned in the water to desirably detect ascending acoustic reflection and/or refraction waves, will also detect undesired descending reflection waves, causing interference in the detection of the ascending waves. This causes certain frequencies of ascending acoustic waves to be selectively reinforced, or attenuated, depending upon the relationship between the distance of the receiver below the sea surface and the wavelength (.lambda.) of the acoustic wave. For example, as shown in FIG. 1, when an acoustic receiver 11 is placed at a depth Z corresponding to 1/4.lambda., an ascending wave 13 of a fundamental frequency f.sub.o, when reflected at the sea surface with a 180.degree. phase change, will produce a descending wave 15 which, upon its arrival back at receiver 11, will be in phase with the ascending wave of the same frequency to reinforce the ascending wave. However, if the acoustic receiver is placed at a depth Z=1/2.lambda., the ascending and descending waves are out of phase thereat, causing attenuation of the fundamental frequency f.sub.o.
The reinforcement phenomenon is repeated at frequencies where the receiver depth Z correspond to 3/4.lambda., 5/4.lambda., etc., i.e., at the odd harmonics of the fundamental frequency f.sub.o, where .lambda..multidot.f.sub.o =C (C=velocity of sound in seawater). Cancellation occurs when the depth Z corresponds to 2/4.lambda., 4/4.lambda., 6/4.lambda., etc., i.e., at the even harmonics of f.sub.o. For receivers located at other depths, there is partial interference between the ascending and descending waves, producing some reinforcement or attenuation, depending on depth and wavelength.
Because of this reinforcement/attenuation phenomenon, when a receiver is placed at or near the sea floor, e.g., on the continental shelf, there will be notches in the spectrum of the received and recorded seismic signal due to the interference caused by the descending reflection wave. This gap in the recorded waves complicates subsequent signal processing and results in the loss of useful seismic information.